The sunspots, Galileo guessed guessed, were clouds floating above the surface, blocking some of the sunlight from reaching us. We now know that sunspots are darker than their surroundings because they are moderately cooler, since their intense magnetic fields somehow slow down the local flow of heat from the Sun's interior. The process which causes this is still unclear.

What is a "magnetic field," anyway?

What follows below is a brief summary of magnetism; more details can be found on the files linked below, all of them parts of the web site The Exploration of the Earth's Magnetosphere. You may look them up--but be prepared to spend extra time!

Just as lines of latitude and longitude help us visualize positions on the Earth's globe, so magnetic field lines (originally named by Michael Faraday lines of force) help visualize the distribution of magnetic forces in 3-dimensional space. Imagine a compass needle which can freely turn in space to wherever the magnetic force tries to point it (such needles exist--see bottom of this web page). Magnetic field lines are then imaginary lines which mark the direction in which such a needle would point.

A compass needle, for instance, has two magnetic poles at its ends, of equal strength, the north-seeking (N) pole and the south-seeking (S) pole, named for the directions on Earth to which they tend to point. Suppose the needle is free to point anywhere in 3 dimensions. If placed near the north pole, it would everywhere point towards the pole, and field lines therefore converge there (see drawing). If placed near the south pole, it would point away from it in all directions, and therefore field lines would diverge there, coming out of the Earth in a pattern that is a mirror-image of the pattern at the north pole. In between the lines form big arches above the Earth's equator, with their ends anchored in opposite hemispheres.

Any bar magnet has a pattern of field lines like that of the Earth, suggesting that the Earth acted as if a short but very powerful bar magnet was inside it. Actually such a magnet does not exist, and the pattern comes from electric currents in the Earth core, and slowly changes, year by year; still, the "terrestrial bar magnet" remains a useful visualization aid.
When two bar magnets are brought together, their (N,S) poles attract each other, their (S,S) and (N,N) poles repel: thus if a bar magnet were hidden inside the Earth, its S pole would be the one that pointed northwards, attracting the N pole of the compass needle. This strange mix-up of terminologies often confuses students: it is best to recognize the mix-up exists and then to ignore it.

Michael Faraday who in the early 1800s introduced the concept of magnetic field lines, believed that space in which magnetic forces could be observed was somehow modified. His was a somewhat mystical view, but later mathematical developments found it quite useful, and today we refer to such a region of space as a magnetic field.

Sunspots were studied by Scheiner and Galileo in the early 1600s (for a detailed but long account, see here), and then a strange thing happened: for about 70 years (1645-1715) they became a rarity. Some speculate that the unusually cold weather during those years was related to their disappearance, but in any case, by the time they returned, the attention of astronomers had moved elsewhere. It was only in 1843 that a German amateur astronomer, a pharmacist named Heinrich Schwabe (Shwah-bay), noted their most famous feature: their numbers grew and shrank, in a somewhat irregular cycle, lasting about 11 years. For the fuller account of Schwabe's discovery,see here.

For a century and a half people looked for a correlation between sunspots and weather (also using tree-rings as evidence for the distant past) and failed to find any. However, sunlight energy-flow is notoriously hard to measure accurately. Even on clear mountain tops the observer is below the ozone layer which absorbs some, and the blue color of the sky shows some light is scattered, and of that, not all ends up going downwards. In the infra-red region, it is hard to separate radiation from the cooling Earth (scattered by greenhouse gases) from the one coming from the Sun.

As example of this problem, have you ever noticed that we are 3% closer to the Sun in January than in July, because of the eccentricity of the Earth's orbit? The difference has been invoked in a theory of ice ages, and observations of the solar constant" in clear air have detected it. However, its variation during the solar cycle amounts to only 0.2%, and to observe it (see graph above) scientists needed observations from spacecraft.

Such observations have now been conducted for about 30 years. Naively, one might expect less sunlight when some of the solar surface is darkened by sunspots. Actually, years of the most sunspots are also peaks of the solar energy output. Perhaps this is caused by increased output in ultra-violet and x-rays in such years, or perhaps heat from the regions beneath sunspots is diverted to neighboring areas of the Sun and radiated from them. Alas, we can only vaguely guess what goes on beneath the visible surface of the Sun.

Note on Solar Magnetic Fields
The fact that sunspots were intensely magnetic was evidence that motions in conducting fluids (like the Sun's plasma) could generate electric currents, whose magnetic field helped maintain the same currents. This made Earth scientists realize that perhaps a similar "fluid dynamo" operated in the Earth's liquid core, and was the cause of the Earth's magnetic field (rather than some strange sort of permanent magnetism). Today "dynamo theory" is well developed, for both Earth and Sun; for more see "The Great Magnet, the Earth," home page http://www.phy6.org/earthmag/demagint.htm.

The most remarkable aspect of such activity is the speed with which it takes place. If a typical big flare spreads over 10,000 km in 10 minutes, it must propagate quite rapidly. Some of its features begin much more abruptly, e.g. the associated x-rays (observable from space) can rise in just a few seconds. All this suggests that the energy source is not the heat of the Sun, which spreads and changes rather gradually, but the intense magnetic fields of sunspots. (For more on these matters, see here.)

Exploring further

A small bar magnet, on gimbals that allow it to point in any direction in space, can be procured from its manufacturer, Cochranes of Oxford, Ltd., Leafield, Oxford OX8 5NT, England. Two types are available, Mark 1 with jewelled bearings for $36.60, Mark 2 with simple bearings for $12.65. For details see their web site: http://www.cochranes.co.uk/secondary.html (scroll down to "Magnaprobe").